Solar panels represent an array of crystalline silicon solar cells. The silicon solar cells are electrically connected in series and convert the incident solar energy into electrical current. Thin strips of tin-lead or tin-silver coated copper that conduct electricity connects between the crystalline silicon solar cells of the array. The strips, usually one to three millimeters wide, are called tabbing ribbons. The tabbing ribbons collect the current from separate crystalline silicon solar cells and conducts the direct current to a solar inverter. The inverter converts the direct current into useable alternating current.
An array of straight, parallel and equally spaced thin, about 30-120 micron wide, current conductors, termed finger lines, usually covers a small portion of the light receiving surface of each individual silicon solar cell. Finger lines reduce the resistance to the photoelectric current and provide lower current losses. Finger lines collect the current from the silicon solar cell surface and transfer it to the tabbing ribbons, through so called bus bars. Each silicon solar cell contains two or three bus bars. which are one to three millimeters wide electric current conducting lines. Typically, the bus bars are parallel to each other and equally spaced. The bus bars are orthogonal to the finger lines array. The tabbing ribbon is soldered to the surface of the bus bars and facilitates transfer of the electric current from the connected silicon solar cells onwards to another silicon solar cell or an accumulator or solar inverter.
Both bus bars and finger lines are usually realized by a single stage of screen printing, which lays down the bus bars and the finger lines across and along the solar cell. More recent methods, deposit the bus bars and finger lines in two successive printing stages. One possible reason for the two stage deposition process is the use of two different materials for the bus bars and the finger lines. Another possible reason is the desire to print bus bars thinner than finger lines, and another possible reason is the use of screen print for bus bars and another printing method (for example, the PTP process by Utilight or stencil print) for deposition of the finger lines. When printing the front side grid in two successive stages, typically, the finger lines are printed and dried first and the bus bars are printed over the finger lines. While a single print step will result in similar heights of the bus bar and the finger lines, in two stage printing, in order to have secured contact between the finger lines and the bus bars, and due to possible miss-alignment between the two patterns, there is a need for an overlap between the two patterns. In the overlap areas of the finger lines and bus bars the height will be different than in the area where finger lines or bus bars only are printed.
The printed height of finger lines is 10 micron to 25 micron: The bus bar height of 5 micron to 25 micron and is sufficient for the subsequent tabbing ribbon soldering process. Common screen printing technology results in bus bars printed over the finger lines. The segments of the bus bars which are printed over the already printed and dried finger lines include bus bar segments with height greater than the rest of the surface of the bus bars. A similar problem could occur when finger lines are printed over previously printed bus bars.
When soldering the tabbing ribbons to the bus bars, such height difference or protrusions can interfere with the tin-lead or tin-silver contact to both surfaces and result in poor soldering and low peel strength.